1. Field of the Invention
The present invention relates to a buzz reduction circuit for eliminating buzz included in a reproduction audio signal of a VCR, and more particularly to a reduction circuit of a vertical period buzz, that is a buzz caused by a magnetic leakage flux of a vertical deflection coil of a television receiver.
2. Description of the Related Art
In the Japanese patent publication No. 10-234005, there is disclosed a method for reducing a vertical period buzz included in a reproduction audio signal of a TV/VCR Combo. FIG. 5 is a block diagram showing a buzz reduction circuit. In an audio output line of a linear audio head 1, there is provided an audio amplifier circuit 2 to amplify a reproduction audio signal from the linear audio head 1 and an audio output amplifier circuit to amplify further the amplified signal. A direct current component is omitted from the output of the audio output amplifier circuit 3 by a capacitor C1, outside noises are omitted by a resistor R1 and a capacitor C2, and finally an audio signal is output under the control of a level of an audio output by a resistor R2.
There is provided a buzz reduction circuit comprising a synchronizing signal separating circuit 4, an integration circuit 5, a phase inversion circuit 6, a waveform shaping circuit 7, and a buzz eliminating circuit 8. An output stage of the buzz eliminating circuit 8 is connected to the rear stage of the audio output amplifier circuit 3. The linear audio head 1 is connected with coils L1 and L2 respectively in series. In order to obtain a reverse phase output that is opposite to a reproduction output from the linear audio head 1 as an output for eliminating a vertical period buzz, the inductances of the respective coil L1 and coil L2 are adjusted suitably and also a mounting angle thereof is adjusted in connection with a deflection coil of a television receiver.
A reproduction video signal from a video head of a VCR is sent to the synchronizing signal separating circuit 4 to separate a synchronizing signal. The vertical synchronizing signal is then extracted in the integration circuit 5, by not letting the horizontal synchronizing signal pass through the integration circuit. The phase of the vertical synchronizing signal is inverted by the phase inversion circuit 6, and the waveform of the signal is shaped by the waveform shaping circuit 7. As a result, a vertical synchronizing pulse is obtained. The buzz eliminating circuit 8 applies the vertical synchronizing pulse only for the vertical synchronizing signal period and reduces a vertical period buzz contained in an audio output from the audio output amplifier circuit 3.
In the above-mentioned conventional technology, the vertical period buzz reduction signal for eliminating a vertical period buzz is inputted to the rear stage of the audio output amplifier circuit 3. However, gain fluctuations of amplifier circuits incorporated in the products influence an audio output, thereby resulting in an uncertainty of buzz elimination. Namely, the buzz component picked up by the linear audio head 1 is amplified by the audio amplifier circuit 2 and the audio output amplifier circuit 3 respectively. As a result, a vibration amplitude of a buzz output from the audio output amplifier circuit 3 becomes larger. When such a vibration amplitude becomes larger, the width of a buzz period portion becomes greater, and as a result, a buzz cannot be eliminated completely by the vertical period buzz reduction circuit.
In order to eliminate variations of a vibration amplitude of buzz to some degree utilizing the coil L1 and the coil L2, it is necessary to adjust the respective coil L1 and coil L2 so that a preferable mounting angle and an inductance are obtained. However, it is troublesome to make adjustments of the coil L1 and the coil L2 in every product, and, furthermore, it reduces production.
In addition, in order to produce a buzz reduction signal to be input to the rear stage of the audio output amplifier circuit 3, it is necessary to shape a waveform of the vertical synchronizing signal. The waveform shaping circuit 7 used for this purpose requires several parts such as an operation amplifier, resulting in an increase in the parts cost and in the production cost.
Instead of the above-mentioned electrical measure, a physical measure can also be adopted to eliminate a vertical period buzz. As shown in FIG. 6, a silicon steel plate 13 used for a magnetic flux shield is inserted between a vertical deflection coil 11 equipped for a CRT 10 of a television receiver and a linear audio head equipped for a VCR 12. Since the silicon steel plate 13 shields a leakage magnetic flux of the vertical deflection coil 11 so as to not jump into a linear audio head, the linear audio head is prevented from picking up a leakage magnetic flux, and thereby a vertical period buzz is eliminated. In this case, however, whenever a layout of the device is changed, it is necessary to review a position and a size of the silicon steel plate. Such review would increase production costs. It should also be noted that a silicon steel plate is a comparatively expensive material. Nevertheless, there is no other substitutive material of low price suitable for eliminating a vertical period buzz.
Where the silicon steel plate is omitted, a vertical period buzz induced by a leakage magnetic flux of a vertical deflection coil equipped for a CRT is output to a linear audio head line. The output vertical period buzz is amplified and finally output as an audio component, so that the S/N ratio deteriorates causing the observer to hear an unpleasant buzzing sound.